1. Field of the Invention
The present invention concerns integrated circuits and pertains particularly to a liquid transfer molding system for encapsulating semiconductor integrated circuits.
2. Related Information
After the manufacture of integrated circuits on a silicon wafer surface, the wafers are typically sawn into individual die. Each die is then attached onto a substrate (for example a leadframe), and bonding wires are used to provide electrical connection from the die to the outside world.
In general, transfer molding is the most widely-used method of encapsulating semiconductor devices because of the versatility and high throughput of such systems. The encapsulation materials used in transfer molding systems normally come in solid form. They are made by pelletizing fine powder of a mixture of resin, filler, hardener, catalyst, carbon black, etc. Some of the disadvantages of using these pelletized mold compounds are dust (escaping from pellets), non-uniform density across the pellets, and moisture absorption.
Liquid encapsulating materials, on the other hand, do not have the problems associated with pageetized powder. They do not produce dust and are very uniform in density and other properties. Most of the encapsulating materials for optoelectronics come in liquid form. Thus they cannot be used directly in a transfer mold system. One way that is commonly used is to turn the liquid materials into solid by partially curing (called B-stage) the materials to a certain degree and forming it into pellet form. However, in this partially cured state, the material has a higher viscosity and thus a degraded flow behavior. In addition, the partially cured state results in poorer adhesion to substrates.
In accordance with the preferred embodiment of the present invention an encapsulation system is used to encapsulate semiconductor products. A bottom mold unit includes a mold pot and a mold piston. A substrate loader loads a substrate into a cavity in the bottom mold unit. A liquid dispenser dispenses encapsulation material into the mold pot. The encapsulation material is in an uncured liquid state when placed into the mold pot. A top mold unit is then clamped to the bottom mold unit.
The present invention allows the use of liquid encapsulating materials to replace the solid mold compounds in many applications. The present invention also facilitates high volume production at lower cost for optoelectronic devices such as light emitting diode (LED) displays or infrared devices.
In addition, the superior adhesion of liquid encapsulating material results in better quality products produced at lower cost than is possible using B-stage (partially cured) materials. This is because additional processes are required to produce B-stage materials, resulting in higher costs of the final product.
Further, using liquid materials opens up new applications for transfer mold such as in the optoelectronics, where many of the optoelectronic devices are encapsulated using cast process. In general, cast process has longer cycle time, lower throughput, and is less automated.
Also, using a liquid materials within a transfer mold system, it is possible to have lower molding temperature if necessary. This is possible because of much lower viscosity of liquid materials as opposed to solid mold compounds, which take relatively more time to melt and have higher viscosity. Besides lower viscosity, liquid encapsulating materials have less air and moisture content as opposed to the pelletized mold compounds. This results in fewer air entrapment or defect in final cured material.
Unlike standard transfer mold, which uses pelletized mold compounds, the use of an automatic liquid dispensing system, as in the present invention, allows the amount of liquid material dispensed to be controlled or adjusted with greater flexibility. This is especially helpful during initial process optimization or debug. The amount of waste material can thus be reduced. Such adjustment is difficult, if not impossible, with pellet mold compounds since that would mean a different pellet size, something that the compound manufacturer would have to provide.